The expression "teeth" in this specification is used for parts of the inside of the mouth which includes one or more teeth and surrounding tissues.
A part of the teeth on which a prosthesis, for example a dental crown, an inlay, a veneer (facet) or a bridge is to be fitted, is suitably trimmed or prepared for that purpose. The dental crown will be fitted on the prepared part of the teeth, for example a molar. For that purpose the inner surface of the dental crown must correspond with the shape of the prepared part of the teeth, taking into account, of course, the bonding agent (cement) to be applied. In order to determine the shape of the inner surface of the prosthesis the shape of the prepared part of the teeth may be established by means of a three-dimensional scanner, which scans the respective part of the teeth. Such a scanner scans the surface in question with, for example, a helium-neon laser beam or an infrared light beam. The reflection of the laser or infrared light beam on the surface can be detected by a so-called CCD (Charged Coupled Device) video camera, after which the detection is digitized and recorded.
It is not unusual to make a replica, made of plaster for example, of the part of the teeth or of the whole set of teeth first. In that case the scanning of the part of the teeth in question may be carried out on the replica. One advantage of using a replica is that a replica is easier for access by the scanner than the teeth themselves.
The prepared part of the teeth blends with a non-prepared part of the teeth, which will not be covered by the prosthesis. This transition--between the prepared part and the non-prepared part--is called the preparation line. In case of a dental crown this line runs around a tooth, generally near the implantation in the jaw.
In principle the preparation line can be perceived as an edge present on the surface of the teeth when scanning the shape of the part of the teeth. Practical tests, however, have shown that it is not possible to determine the preparation line in a sufficiently reliable manner on the basis of the scanner detections. That is why a method has been developed to display the surface detected by the scanner on a screen in such a manner, that the preparation line becomes visually recognizable. After that the preparation line can be established manually and be recorded in the scanner (or computer) memory by moving a pointer across the screen--along the preparation line--by means of a mouse.